Advances in one area of technology often require commensurate advances in supporting technologies to realize the full benefit of the advance. For example, observers of the microprocessor industry are familiar with “Moore's Law,” which posits that the complexity of semiconductor devices doubles every two years. Microprocessor development arguably represents the most dramatic illustration of Moore's Law. Pioneering microprocessors released in the 1970's operated at clock speeds well under 500 KHz, and included fewer than five thousand transistors. Modern microprocessors operate at clock speeds in excess of 1 GHz and include millions of transistors. Exploiting these dramatic gains required advances in a host of supporting technologies, from advances in memory technology and circuit fabrication, to advances in power supply design.
Indeed, modern microprocessors could not provide their dramatic performance gains absent today's sophisticated power supplies. For example, high-end microprocessors can consume in excess of 80 Watts of power and operate at 2 VDC or less. These requirements translate into power supply output current requirements in excess of 40 Amps, yet the power supply must maintain tight output voltage regulation, even when faced with dramatic step changes in output current. In general, modern electronic systems require responsive power supplies capable of providing relatively clean power at well-controlled voltages, over a wide range of quickly changing load conditions.
Linear regulators are relatively responsive, accurate, and low noise. Such regulators operate using controlled impedance, typically implemented as a pass transistor, to create a voltage drop across the controlled impedance such that the input voltage minus the voltage drop equals a desired output voltage. With proper use of load capacitors, linear regulators offer good line and load regulation performance, with virtually no noise problems. However, linear regulators are inefficient when required to regulate to an output voltage significantly below their input voltage. Because of high currents required by modern electronic systems, main power supplies often operate at 12 VDC or 24 VDC. Regulating such primary voltages down to 2 VDC, or even 5 VDC, for high-current loads is impractical using linear regulation.
Switch-mode power supplies offer significant efficiency advantages compared to linear regulation, and avoid most of the power dissipation problems associated with linear regulators. Switch-mode power supplies operate, as their name suggests, by switching some type of reactive element in and out of a supply path to effect output voltage control. The reactive elements may be one or more capacitors, such as in a charge-pump type switcher. However, inductor-based switch-mode power supplies are more common in high-current, high-performance power supply applications. Typically, an inductor is switch-connected to a voltage source at one end, and to an output load at the other end. An output capacitor sits in parallel with the output load. A switch controller rapidly connects and disconnects the inductor to the voltage supply to regulate the load voltage. The output capacitor serves as a low-impedance current source to the load, and helps smooth the output voltage of the power supply.
The switch controller in a switch-mode power supply requires some form of feedback to effect closed loop voltage regulation on the load. Many different regulation topologies exist, including voltage-mode feedback and current-mode feedback. In both voltage-mode and current mode feedback, an error amplifier typically generates a control signal by amplifying a difference between a feedback signal and a reference signal. This error amplification can reduce the bandwidth of the feedback loop, diminishing the switch controller's ability to respond to highly dynamic load changes, as are common with microprocessors and other high-performance electronic circuits.
Thus, it is with respect to these considerations and others that the present invention has been made.